Antagonistic effects of arginine methylation of LSM4 on alternative splicing during plant stress responses

Arabidopsis PROTEIN ARGININE METHYLTRANSFERASE 5 (PRMT5) post- translationally modifies RNA-binding proteins by arginine (R) methylation. The impact of this modification on the regulation of RNA processing is largely unknown. Here we use LSM4, a component of the spliceosome, as a paradigm to study the impact of R-methylation on its function in RNA processing. We identify in vivo targets of LSM4 and show that LSM4 regulates alternative splicing of a suite of them. Furthermore, LSM4 affects mRNA levels of some of the targets, showing for the first time its role in both AS and steady-state abundance. The lsm4 and prmt5 mutants show a considerable overlap of genes with altered splicing patterns, suggesting that these might be regulated by PRMT5-dependent LSM4 methylation. Wild-type LSM4 and an unmethylable version complement the lsm4-1 growth and circadian rhythms defects, suggesting that methylation is not critical for growth in normal environments. However, LSM4 methylation increases with ABA and is necessary for plants to respond properly to salt stress. In contrast, LSM4 methylation is reduced by bacterial infection, and plants expressing unmethylable LSM4 are more resistant than plants expressing wild-type LSM4. This tolerance correlates with decreased intron retention of immune-response genes upon infection, augmenting the functional isoform. Taken together, this provides the first direct evidence that R methylation adjusts LSM4 function on pre-mRNA splicing in an antagonistic manner in response to biotic and abiotic stress. ### Competing Interest Statement The authors have declared no competing interest.